There have been five dramatic leaps in electronic technology during the past hundred years. The first four include the light bulb, electron tube, transistor, and integrated circuit. The most recent jump, and perhaps the most significant, has been the emergence of the microprocessor in the 1970's and 1980's. A microprocessor and associated memory and interfacing components form a "microcomputer", a physically small digital machine as powerful as a room-sized computer of only two decades ago. These microcomputers have found almost limitless applications from electronic games, calculators, microwave ovens, and point of sale terminals, to traffic signals, automobile ignition controls, copying machine controls, and deep space probes. The microcomputer has been a leading factor in the proliferation of automated processing controls including industrial robots. However, there have been few microprocessor applications to electric power transmission and distribution systems. In particular, there has been nothing comparable to automated control of the protective and operating apparatus used on power systems. The emergence of the microcomputer provides the technical capability provided the specialized nature of electrical power distribution systems and their controls is fully understood.
In most electrical power distribution systems the voltage level would tend to vary due to several factors such as load, line inductance, or line resistance. This variation is disagreeable to the customer since it could result in poor performance or even equipment damage. A step-voltage regulating transformer is a device which is often used to maintain the voltage of an electrical distribution system or network relatively constant. The voltage is maintained relatively constant by an apparatus which: (1) detects changes in the system voltage; and (2) automatically adjusts system voltage without interrupting service. An early step voltage regulating transformer is disclosed by Sealey in U.S. Pat. No. 2,713,142.
For the most part, voltage regulating transformers are tapped autotransformers consisting of: a tapped series winding that facilitates plus or minus 10% regulation; a shunt winding across the regulator output terminals; a potential or voltage sensing winding closely coupled to the shunt winding; and a current transformer primary winding in the load line at the output terminal. A reversing switch is also provided which is always in a neutral position or a "raise" or a "lower" position, depending on whether the regulator is used to boost or buck the source voltage. The reversing switch is disposed across the ends of the series winding. Under this arrangement with the reversing switch in the raised position, the series winding becomes additive with respect to the shunt winding as the number of turns placed in series with the load increases. Therefore, the amount of voltage boost increases. When the reversing switch is moved to the lower position, the series windings, therefore, become subtractive with respect to the shunt winding and the amount of voltage buck depends upon the number of turns placed in series with the line.
Typically, an automatic control device is provided to change the tap settings. For the most part, these automatic controllers are not responsive to voltage changes due to current flowing both into and out of the input terminals of the transformer. Those skilled in the art know that in the case of multiple feed systems or feed systems employing alternate power sources, it is possible for a reverse power flow to occur. Unless the automatic voltage regulating portion of a transformer is arranged to be responsive to current flowing in either direction, instability is likely to occur. The traditional solution to this problem was to use a separate potential transformer across the input terminals of the regulating transformer and to use it to sense changes in the direction of current flow. those skilled in the art know that those control devices, for the most part, are electro-mechanical in nature, are difficult to adjust and maintain in alignment, and relatively expensive to produce, especially if they are to have a reverse current or reverse power sensing capability. The mechanism described by U.S. Pat. Nos. 2,280,766; 2,009,383; and 2,381,271 are representative of voltage regulating transformers totally using electromechanical devices for control.
Bearing in mind the recent and dramatic progress that has been made with the use of microprocessors, a modern voltage regulating transformer design is long overdue. A relatively inexpensive control which can be readily adapted to existing distribution transformer voltage regulating designs in such a manner that the automatic regulating circuitry of the device includes the capability of sensing voltage changes due to current flowing both into and out of the transformer output terminals would be welcomed by both electrical utilities and their customers. Preferably, existing proven autotransformer windings and tap changing mechanisms should be used to the maximum extent practicable. Such a voltage regulating transformer design would not only reduce the overall cost of manufacture, but also the operating cost of maintaining the transformer throughout its life. Moreover, if such a control incorporates modern, digital communicating schemes, the capability would be provided for electrical utilities to automate their distribution systems and in the process improve the systems overall efficiency. Under this arrangement, a master substation computer could send signals automatically throughout the distribution system to the digitally controlled equipment. Since each voltage regulating transformer uses a self-sufficient microprocessor or microcomputer, the intelligence of such an automated distribution system is dispersed and close to the source of problems and customer requirements.